Moving target detection processing is known as standard processing of a radar apparatus. Since a radar received echo includes a target reflection signal and unnecessary reflection signals (which are called clutter) from grounds and the like, this moving target detection processing is for suppressing the clutter by discriminating the target reflection signal from the clutter on the basis of the moving speed of a target. Here, a filtering process is performed on Doppler frequencies generated in relation to the moving speed of the target.
Typical examples of this filtering process for the Doppler frequencies include MTI, FFT, and the like. However, in order to obtain a signal process gain by this Doppler filtering process, a transmission frequency at the time of receiving a signal used for the Doppler filtering process must be coherent (i.e., phase-continuous at the same frequency) and transmission pulse intervals must be constant.
Meanwhile, a target search by the radar is performed by sequentially transmitting transmission pulses to a necessary range all around an antenna while rotating the antenna. Because the antenna is rotated, a time period for irradiating an intended target with the transmission radio waves is limited. The number of transmission pulses to be transmitted within this irradiation period is called a hit number. The above-described Doppler filtering process is performed on transmission pulses within a time period called a CPI (coherent processing interval) having this hit number as an upper limit, or in other words, on the same number of received echoes.
As described above, in a certain number of transmission pulses known as the CPI, the transmission frequency needs to be constant and the transmission pulse intervals also need to be constant. However, as a result of processing such transmission pluses through the filtering process for suppressing the clutter, there occurs a problem of having a speed range where it is impossible to detect the target (which is called blind speed) (see FIGS. 12-15 and 16 in Non Patent Literature 1, and FIG. 4.16 (a) and (b) in Non Patent Literature 2).
To solve this, a technique called staggering for eliminating the blind speed by sequentially transmitting multiple CPIs having different transmission pulse intervals has been generally employed (see FIGS. 12-17 in Non Patent Literature 1 and FIG. 4.16 (c) in Non Patent Literature 2).
Here, Patent Literature 1 discloses a conventional technique concerning the above-described measure. A radar signal processing apparatus disclosed in this Patent Literature 1 includes: a MTI map generator configured to set up an MTI map in a fixed clutter region; an automatic clutter map generator configured to set up an automatic clutter map in a moving clutter region by use of an inputted video signal; a synchronizer configured to output trigger pulses for defining a staggering cycle, the trigger pulses having sampling intervals determined at least based on a Doppler characteristic of a target and on a hit number; a filtering coefficient controller configured to receive the MTI map, the automatic clutter map, and the trigger pulses and to output a predetermined different filtering coefficient in response to each of the trigger pulses; multiple filters set to have filtering characteristics based on the predetermined filtering coefficient outputted from the filtering coefficient controller; multiple clutter suppressors configured to perform clutter suppression processing on outputs from the multiple filters; and a greatest value selector configured to select the greatest value among outputs from the multiple clutter suppressors. The greatest value selector receives an output signal from the predetermined filter which is different for each trigger pulse, selects the greatest output signal, and synthesizes a radar signal to be outputted. In short, the radar signal processing apparatus reduces an influence of MTI blindness in the multiple CPIs having different transmission pulse intervals by appropriately changing coefficients for Doppler filter banks for the CPIs. In this way, it is possible to obtain a target signal at a high detection probability.
As a technique related to the conventional technique, Non Patent Literature 3 discloses a technique for the thinning-out of the transmission pulses to be described later, for example. Non Patent Literature 3 discloses that it is possible to form Doppler filter banks even when a certain number of pulses are thinned out of a transmission pulse train with constant intervals. According to Non Patent Literature 3, use of thinned-out transmission pulse trains for tracking the respective targets enables tracking of multiple targets at the same time, and thereby reduces necessary time.